In a terrestrial television broadcasting system, a relay device is installed in a relay station to send out a broadcast radio wave to regions such as a mountainous region in which it is difficult to receive the broadcast radio wave. In this relay device, a broadcast radio wave transmitted from a parent station is received, amplified and again sent out.
Also, the adoption of an orthogonal frequency division multiplex (OFDM) method is determined in Europe and Japan as a transmission method of digital terrestrial broadcasting. In this OFDM method, because a plurality of signals having the same frequency as each other can be used in an area, a single frequency network (SFN) can be used.
Therefore, it is studied to prepare the SFN in the terrestrial digital broadcasting, and it is studied to use the reception frequency and the transmission frequency having the same value as each other in a relay device so as to efficiently use the frequency. In cases where the reception frequency and the transmission frequency in a relay device of a relay station are the same as each other, both a radio wave transmitted from a parent station and a radio wave transmitted from the relay station are received in a reception antenna of the relay device. In this case, there is a possibility that a signal is oscillated in an amplifying unit of the relay device and a broadcast service cannot be performed.
To prevent the oscillation of the amplifying unit of the relay device of the relay station, it is required that a received electric power D of the radio wave received in the parent station and a received electric power U of a transmission radio wave (or a go-around radio wave) of the relay station satisfy a condition D>U. However, because the electric power of the radio wave transmitted from the relay device is sufficiently higher than the electric power of the radio wave received in the relay device, it is not generally easy that the condition D>U is satisfied. Therefore, various methods have been proposed to remove the go-around radio wave and to sufficiently reduce the received electric power U of the transmission radio wave of the relay station.
As a method of removing the go-around radio wave by using a reception antenna, a method of canceling-out the go-around radio wave by using a plurality of reception antennas has been proposed. FIG. 1 is a block diagram showing the configuration of a conventional relay device disclosed in Published Unexamined Japanese Patent Application No. H11-298421 (1999).
In FIG. 1, 1 indicates a main reception antenna for receiving both a radio wave (of a frequency f1) transmitted from a parent station and a go-around radio wave of a relay station in which the relay device is installed. 2 indicates a subsidiary reception antenna for receiving the go-around radio wave of the relay station. 3 indicates a changeable attenuator for changing an amplitude of a reception signal of the go-around radio wave received in the subsidiary reception antenna 2 by a preset attenuation factor. 4 indicates a phase shifting unit (or phase shifter) for changing a phase of a signal output from the changeable attenuator 3 by a preset shifting phase value. 5 indicates a composite signal producing unit (or composite signal producer) for producing a composite signal from both a signal of the radio wave of the parent station and a signal of the go-around radio wave received in the main reception antenna 1 and the signal sent from the phase shifting unit 4. 6 indicates an amplifying unit for amplifying the composite signal produced in the composite signal producing unit 5. 7 indicates a transmission antenna for transmitting a transmission radio wave (of the frequency f1) of the relay station according to the composite signal amplified in the amplifying unit 6.
Next, an operation will be described below.
The main reception antenna 1 has a directivity in the direction of the parent station, and the radio wave transmitted from the parent station is received in the main reception antenna 1. Also, a go-around radio wave transmitted from the transmission antenna 7 of the relay station is mixed with the radio wave of the parent station and is received in the main reception antenna 1. A reception signal composed of a mixture of the radio wave of the parent station and the go-around radio wave is received in the composite signal producing unit 5. The subsidiary reception antenna 2 is directed towards the transmission antenna 7, and the go-around radio wave transmitted from the transmission antenna 7 of the relay station is received in the subsidiary reception antenna 2. An amplitude of a reception signal of the go-around radio wave is changed by a preset attenuation factor in the changeable attenuator 3. A phase of a signal received in the phase shifting unit 4 is changed by a preset shifting phase value, and the signal is output to the composite signal producing unit 5.
In the composite signal producing unit 5, a composite signal is produced from a signal of a mixture of the radio wave of the parent station and the go-around radio wave received in the main reception antenna 1 and a signal of the go-around radio wave which is received in the subsidiary reception antenna 2 and of which the amplitude and the phase are adjusted. In this case, the attenuation factor of the changeable attenuator 3 and the shifting phase value of the phase shifting unit 4 are set so as to produce a composite signal from a signal of the go-around radio wave received in the main reception antenna 1 and a signal of the go-around radio wave received in the subsidiary reception antenna 2 having the same amplitude as each other at phases opposite to each other. Therefore, the signal of the go-around radio wave is canceled out in the composite signal, and the output of the composite signal producing unit 5 is composed of the composite signal of only the radio wave component of the parent station.
Because the conventional relay device has the above-described configuration, when characteristics of the main reception antenna 1, the subsidiary reception antenna 2 and/or the transmission antenna 7 are changed due to a change of the environment caused by wind, snow or the like, the two go-around radio waves received in the composite signal producing unit 5 do not have the same amplitude as each other at the phases opposite to each other. Therefore, a problem has arisen that the go-around radio wave cannot be completely removed from the composite signal.
Also, in cases where the conventional relay device is set so as to be automatically adapted for the change of the environment, it is required to distinguish the go-around radio wave from the radio wave of the parent station. Therefore, as is described in the patent application, another problem has arisen that it is required to superpose a relay station identification signal on the transmission radio wave of the relay station transmitted from the transmission antenna 7.
The present invention is provided to solve the above-described problem, and the object of the present invention is to provide a relay device in which a go-around radio wave can be canceled out without the superposition of a relay station identification signal while being adapted for a change of the environment caused by wind or snow.